Typically, the furnace walls are lined with water tubes, pipes through which water is circulated, heated and converted into steam and collected in a large drum usually above the furnace. In order to heat large quantities of water, multiple burner assemblies can be employed which are inserted through at least one wall of the furnace or at each of the corners or roof mounted, depending upon the designs of the manufacturer. The burners are at least positioned so that combustion occurs at or near the center of the furnace interior so that the heat is more evenly spread.
The coal fuel is pulverized before it is fed into the furnace through a coal nozzle. There, combustion is started with a smaller gas or oil flame which provides the necessary combustion temperature and input to ignite the coal. In addition to the air used to carry the pulverized coal, other air is supplied through an air or wind box and is circulated within a larger conduit through which the burner assembly is positioned. This air enters the furnace circumferentially around the pulverized coal to provide a source of air for proper combustion. Additionally, more air can be fed into the furnace from other sources.
The air is intended to insure complete as well as controlled combustion of the coal. Controlled combustion means that the flame should begin at or near the mouth of the coal nozzle and extend to the center of the furnace interior in a large, bushy shape. When the combustion is not properly controlled, the flame may not begin until some distance from the tip and then it may burn with a narrow shape appearing more as a jet or torch. When this occurs, some quantity of the coal is not combusted and it will either fall to the furnace floor or be carried through the furnace and become deposited on various heat transfer surfaces. The latter creates the potential for catastrophic air heater fires and the like. As to the combusted quantity, it may provide a flame beyond the center of the furnace which provides uneven heating and in extreme occasions it could focus on several of the tubes on the far wall which can lead to premature failure. In addition, it is known that a narrow, pencil-like flame cannot provide the same amount of energy as a large bushy flame.
Current operating practice of existing coal burners involves supplying the total amounts of fuel and air to the furnace. Individual control of fuel and air to groups of burners is made through macroscopic adjustments to the pulverizer capacity, main forced draft fan flows and the like. In an attempt to mix coal and air properly, movable register doors of various design and configurations have been employed. These register doors are generally located away from the point of coal entry into the furnace and essentially function as an on/off gate for the flow of air to any given burner and are not a control device. Existing burner assemblies have used various combinations of air supplies, conduits and swirling devices in an attempt to create a structured turbulence that encompasses the pulverized coal and passes between the particles to provide sufficient air at each coal particle in the expectation that complete and maximum combustion will occur. Nevertheless, complete and efficient combustion can only be achieved by supplying the correct amounts of air and fuel which must undergo proper mixing at specific locations within the coal burner/furnace interface.
Despite the years that burner assemblies have been employed and the many design variations, flame adjustment or control can take much time or coal fuel is wasted and in some instances, maximum combustion efficiency is never obtained. More complex assemblies may increase some segments of combustion efficiency but these are more labor intensive and require longer down time of the furnace when work is required on the various components which must be periodically removed, disassembled and cleaned or replaced.